1. Field of the Invention
The present invention relates to wind turbines, and more specifically, to a wind flow estimator for a wind turbine having a rotor driven by wind applying power to turn rotor blades.
2. Description of Related Art
Most existing wind turbines operate at constant speed. The rotor drives a generator, such as an induction generator, at a constant rotational speed. Constant speed operation is required for a direct link with a constant frequency electrical utility grid. Other more modern wind turbines operate at variable speed to produce DC power, which a converter changes to AC power synchronous with the attached power grid.
Constant speed turbines adapt to increasing wind speed by detecting an increase in rotor rotation rate and then increasing the power generated. This increases the load torque on the rotor and keeps it from speeding up. If this is insufficient, or causes unacceptable power fluctuations, the pitch of the rotor blades is changed to reduce the rotor torque provided by the wind. If the rotor speed is not in sync with the grid, power is not generated.
Variable speed turbines generate power at all wind speeds, but have favored operating points at each wind speed that generate optimal power without over-stressing the components. Striving to follow the operating points as wind speed increases, these turbines also sense rotor speed and use load torque and rotor blade pitch for control.
Control strategies based on detecting changes in rotor speed are limited in their effectiveness due to the substantial time lag between wind changes and rotor speed changes. Direct knowledge of wind flow is central to better control of the wind turbine and a means to estimate wind flow and track it over time provides for improved control. Current patents in this field include those of Holley (U.S. Pat. No. 5,155,375 and U.S. Pat. No. 5,289,041) in which the rotor rotation rate, blade pitch angle, wind torque and generator torque are combined to estimate wind flow.
To accurately track wind flow, it is necessary to determine the average wind flow over the area swept by the rotor blades of the wind turbine. Anemometers installed near to the area swept by the blades cannot accurately measure the average wind speed because they measure wind speed at a single location, whereas wind speed may vary over the area swept by the blades. Further, the blades after the wind pattern and remove energy from the wind, and a sensor behind the blade will not reflect the wind in front.
A rough estimate of the wind speed can be obtained by measuring rotor speed and electrical output power. Since forces other than the wind influence these parameters, this method is not sufficiently accurate.
In addition to controlling the turbine, it is desirable to damp the oscillations of the tower. The tower moves in response to the wind on its structure and on the rotor blades. Tower motion is inherently lightly damped and can oscillate substantially. Tower life is shortened by the associated mechanical flexing.
U.S. Pat. No. 4,435,647, to Harner, et al discloses tower-mounted wind turbine powered electric generating system, having a control to modulate rotor blade angle to maintain rated torque or power in turbulent wind. The control provides a blade angle command component, which adjusts the turbine rotor blade angle in a manner to provide aerodynamic damping of the tower primary resonant frequency in response to a predicted motion signal. The predicted motion signal is indicative of analytically anticipated longitudinal motion of the tower parallel to the rotor axis, as a filtered function of the blade angle reference signal.
U.S. Pat. No. 4,420,692, to Kos, et al discloses an accelerometer disposed on the support tower of a wind turbine electric generating system in the vicinity of the rotor. The accelerometer provides a motion signal indicative of acceleration of the tower in the direction of the rotor rotational axis. The signal is passed through a band-pass filter. A torque/power controlled blade pitch angle reference signal is generated for constant torque/power in response to wind turbulence. The band-pass filtered accelerometer signal is added to the blade reference signal and used to control the pitch angle of the rotor blades through a pitch change mechanism. This provides additional, positive aerodynamic damping to the tower by modulating the blade angle chosen for constant torque/power in response to wind turbulence.
The Holley patents (U.S. Pat. No. 5,155,375 and U.S. Pat. No. 5,289,041) estimate wind speed but not from tower measurements.
The Kos, et al and Harner, et al patents (U.S. Pat. No. 4,420,692 and U.S. Pat. No. 4,435,647) are concerned only with tower damping using measured acceleration through a bandpass or other simple filter. No attempt is made to estimate wind flow from tower measurements.
It is desirable to provide a turbine control system including a wind or water flow estimator that estimates and tracks wind or water flow using the motion of the turbine support structure, such as a tower or underwater tether, as it moves in response to the flow. The flow estimator results are then used in the turbine control system to properly adjust its operating point, to tune the controller, to control the rotor rotation rate, and to damp support structure oscillations.